In a wire or wireless transmission system through which a multi-valued signal is transmitted, because its transmission path and transmitting/receiving circuits are sensitive to any variation of ambient temperature, power supply voltage or the like, the amplitude and DC center level of the multi-valued signal which is inputted into an encoding circuit as an input baseband signal is changed. Since such changes are directly connected to the decease of noise margin, large changes cause an error in decision in the decoding circuit.
Therefore, it is desired to make such circuit design and mechanical design that an influence due to extrinsic factors such as ambient temperature, power supply voltage, etc. can be avoided. However, to attain such desire brings about the increase of production costs inevitably.
Another attempt was to apply an automatic amplitude controlling circuit for coping with the amplitude change and an automatic DC center level correcting circuit for coping with the DC center level change. In general, this arrangement includes a negative feedback control loop which operates with a specified signal, for example, a bit synchronization signal, etc. included in the header portion of the packet. Therefore, in order to stabilize the operation of this loop, a loop operation convergence period of time is needed. Moreover, this operation must be completed before the multi-valued signal which is data information starts. Accordingly, this system requires long training period of time for higher stability and accuracy of the automatic control loop, which deteriorates transmission efficiency.
Further, there is a method of detecting the DC center level and amplitude of the bit synchronization signal from one or two cycle component thereof using the fact that the bit synchronization signal is transmitted generally in the form of a sinusoidal waveform, and of generating reference voltages for the decoding of the multi-valued using the detected DC center level and amplitude of the bit synchronization signal. This method can obtain the reference voltages which vary in accordance with the variation of the input signal, and can eliminate the disadvantage that the transmission efficiency is decreased because the period of time necessary to produce the reference voltages is extremely short. However, in this system, an integration circuit and detection circuit are used without a negative feedback control loop, and therefore, these circuits must be designed and constructed in view of high stability and accuracy. This also causes the increase of production cost.